Alkali Metals Doping for High-Performance Planar Heterojunction Sb2S3 Solar Cells

Sb2S3 as a stable light harvesting material has received increasing attention for solar cell applications. To improve the device efficiency, much effort has been put into the materials synthesis, interfacial engineering, and device structure design. Here, it is demonstrated that doping of alkali metal (Li, Na, K, Rb, Cs) ions essentially affects the morphological, crystal, optical and electrical properties of Sb2S3 films. The structural and compositional analysis shows that the doping is in form of alkali metal-sulfur chemical bond at both surface and grain boundaries of Sb2S3 films. Compared with the pristine Sb2S3, we observe that Li and Na doping are not able to improve the device efficiency, while K, Rb, and Cs notably increase energy conversion efficiency. The most effective enhancement is found in Cs-doped Sb2S3, where the carrier concentration, crystallinity, and film formability show remarkable improvement. The device based on the Cs-Sb2S3 film delivers a power conversion efficiency of 6.56%, which is the highest efficiency in planar heterojunction Sb2S3 solar cells, regardless of whether the films are fabricated by a solution process or thermal deposition. This research identifies that doping of heavy alkali metals is an effective approach to improve the performance of Sb2S3 solar cells.